Automatic merge from submit-queue (batch tested with PRs 45364, 45369, 45323)
kubeadm: edit apiserver flags for storage-backend
**What this PR does / why we need it**: Since storage-backend defaults to etcd3 in 1.6 (see https://github.com/kubernetes/kubernetes/pull/36229), no need to have a flag defining the default.
**Special notes for your reviewer**: /cc @luxas
**Release note**:
```release-note
NONE
```